The present invention relates generally to spark plugs, and specifically to the placement of electrically insulating materials as a film or integrally bonded component of igniter electrodes for the purpose of controlling spark discharge location and electrode material erosion from spark discharges.
In today's jet engines increasing use is made of the ignition system which is energized continuously or by electronic controls, under conditions of high temperatures and pressures, for purposes of providing automatic relight in event of flameout. Energizing ignition under these conditions (while combustion is active) compresses the spark discharge into small, confined volumes within the igniter tip such that "undercutting" of the electrode shell occurs.
The task of controlling the spark discharge location and reducing electrode material erosion is alleviated, to some degree, by the following U.S. Patents, which are incorporated herein by reference:
U.S. Pat. No. 3,883,762 issued to Harris on 13 May 1975; and PA1 U.S. Pat. No. 4,337,408 issued to Sone et al on 29 June 1982.
Harris et al show an igniter plug intended for use in an aircraft jet engine. In this patent the insulator between the center electrode firing tip and the ground electrode has semiconducting metal oxide coating formed on it.
In Sone et al an insulator is located between a recessed center electrode and an outer electrode. The Sone et al insulator carries a film which may be formed by ion plating or sputtering or plasma spraying. Disclosed compounds for film are silicon dioxide and silicon carbide.
In view of the foregoing discussion, the need remains for igniter which directs the spark discharge to prevent undercutting under active combustion conditions. The present invention is intended to satisfy that need.